PETG Infill Problems: Fix Broken Lines, Gaps, and Weak Top Support

The outside of your PETG print looks clean. Inside, however, the infill contains thin strands, missing sections, or loose connections. When the top layers begin, those hidden defects become a rough or sagging surface.

These PETG infill problems are easy to overlook because the outer walls can remain acceptable while the interior prints under different conditions.

Before increasing infill density, determine whether the printer is failing to deposit the intended lines—or whether the intended structure simply provides too little support.

Start by Comparing the Preview with the Print

Inspect the sliced model at the height where the problem first appears.

Sparse infill is supposed to contain empty space. Some patterns also change shape across layers. A gap is only evidence of a printing problem when material is missing from a path the slicer actually planned.

What you observe	What to investigate first
Intended lines are thin or interrupted	Extrusion demand and material delivery
Gaps occur mainly at line beginnings	Restart behavior and infill anchoring
Infill does not connect properly to walls	Connection settings and local extrusion quality
Infill is complete, but the roof sags	Internal spacing and top-shell thickness
Walls and infill both deteriorate	A broader extrusion or feeding problem

Keep a photograph of the exposed infill before it is covered. This is often more informative than examining the finished top surface alone.

1. Broken Infill Lines: Check the Demand on the Hotend

Outer walls and infill may use different speeds, line widths, and layer heights. A successful outer wall therefore does not prove that the printer can deliver the material required by every internal path.

Maximum volumetric speed limits how much material the printer is asked to extrude per second. It works alongside movement speed rather than replacing it. Prusa maximum volumetric speed guide

For an approximate illustration:

Volumetric demand ≈ line width × layer height × movement speed

At a nominal width of 0.45 mm, height of 0.20 mm, and speed of 100 mm/s, the estimate is 9 mm³/s. Doubling the speed doubles that estimate.

This is an illustrative calculation, not a recommended PETG limit. Slicers may calculate extrusion cross-sections more precisely, and the workable limit depends on the material and hardware.

What to test: reduce infill speed while leaving the walls and other settings unchanged. Inspect the actual speed or flow preview after slicing.

If the internal lines become continuous, investigate the material’s workable extrusion demand. Do not immediately compensate by increasing the global flow ratio.

2. Check Whether Infill Uses a Different Layer Height

Some slicers can combine infill across multiple layers to reduce printing time. That changes the thickness of the internal extrusion even when the visible outer layers remain fine.

Prusa documents this option as a way to use different layer heights for infill and perimeters. Prusa infill settings

If the problem began after enabling a faster profile, inspect this setting.

What to test: return infill to printing every layer, using the appropriate nozzle profile, and compare the same section.

Do not judge the result only by the nominal movement speed. A thicker extrusion at the same speed can demand more material.

3. Gaps at the Beginning of Lines: Inspect the Connections

If most of each line looks healthy but its beginning is missing, the problem differs from a line that remains thin throughout its length.

Check whether the gap follows a travel move or retraction. Inspect how the slicer connects the infill to the surrounding walls.

Prusa’s infill documentation describes anchoring as a way to stabilize the beginning of an infill line, including for PETG. Settings and terminology vary between slicers. Prusa infill anchoring guidance

What to test: restore a compatible material preset if retraction or anchoring has been heavily customized. Then adjust one relevant setting at a time.

Avoid increasing infill overlap indiscriminately. A larger overlap changes material placement near the walls and may introduce visible ridges or other surface defects.

Use a small sample with enough internal line starts to reproduce the symptom.

4. Complete Infill, Weak Top Surface: Check Support and Shell Thickness

If the infill lines are continuous and correctly placed, the next question is whether they adequately support the roof.

Wide spaces require the first top layers to span larger gaps. A thin top shell may not fully conceal the irregular surface formed during that transition.

What to test:

• Review the distance between support points beneath the top.

• Check total top-shell thickness, not just the number of layers.

• Test a modest density increase or another suitable pattern.

• Compare the roof after changing only one factor.

Layer count alone can be misleading: the same number of top layers produces a different total thickness when layer height changes.

Do not use extra top layers to hide broken infill before correcting the extrusion problem underneath.

5. Why More Density Is Not the First Fix

Infill density describes how much internal structure is planned. It does not guarantee that the printer deposits that structure correctly.

Increasing density can shorten unsupported spans beneath a roof. It also adds material, paths, and printing time.

If existing lines are incomplete, adding more intended lines may simply create more incomplete extrusion.

Separate the two decisions:

First: Can the printer produce the selected pattern reliably?

Then: Does that pattern and density meet the part’s support and performance needs?

Similarly, changing to Gyroid or Cubic may suit a particular model, but neither pattern repairs a feeding restriction or an excessive extrusion demand by itself.

6. When to Investigate More Than Infill Settings

Broaden the investigation if walls also develop gaps, extrusion becomes inconsistent at conservative settings, or the extruder repeatedly slips.

Check the filament route, spool movement, nozzle condition, and material profile. Evaluate moisture-related concerns using the filament manufacturer’s drying instructions rather than assuming every weak internal line needs drying.

Increasing temperature should also remain within the exact product’s recommended range. It is not a substitute for checking the feeding system or selecting a workable extrusion demand.

Make a Useful Test Sample

Use a short model that preserves the original internal span and infill path length. An extremely small cube may never reproduce the demanding section.

For an inspection-only test, you can omit the top shell to expose the interior. Follow it with a closed-top version when testing roof quality.

Test	Keep unchanged	Evaluate
Lower infill speed	Material, pattern, density, and geometry	Line continuity
Disable combined infill layers	Other profile settings	Internal deposition consistency
Adjust anchoring	Speed and material condition	Connections at line starts
Change top-shell thickness	Reliable infill beneath it	Roof closure and surface quality
Increase density	Extrusion settings	Support beneath the roof

Save both the result and the settings. A successful test should explain what improved, not merely produce a better-looking object.

For future projects, browse the INKCLOUD filament collection and check the guidance for the specific PETG product. Build a reliable profile before optimizing print time.

Frequently Asked Questions

Why are my PETG walls good but the infill weak?

The infill may use different speeds, widths, or layer heights. Compare the actual extrusion conditions before blaming the spool.

Should I increase infill flow?

Only after identifying a deposition problem that justifies it. A global increase can affect walls that already print correctly.

Is 100% infill the strongest solution?

It is not a universal solution. Part geometry, walls, orientation, and layer bonding also matter.

Can a clean top surface hide poor infill?

Yes. A satisfactory exterior does not verify the internal structure or the part’s performance.